U.S. patent application Ser. No. 11/829,050, filed Jul. 26, 2007 and published as Pub. No. US 2008/0026162 of Dickey et al. (“the '050 application”) describes various methods and systems for radical-enhanced atomic layer deposition (REALD). The specification of the '050 application, which is incorporated herein by reference in its entirety, describes deposition methods involving alternating exposure of a substrate to a first precursor gas and a radical species, wherein the radical species is generated in-situ by an excitation source such as a steady-state direct-current (DC) or radio-frequency (RF) plasma generator. The first precursor gas is introduced at a location spaced apart from and generally downstream from where the radical species is generated, to provide a radical deactivation zone therebetween. In some embodiments disclosed in the '050 application, the plasma generator generates a direct plasma proximal of the substrate surface from a purge gas flowing through the system, wherein the purge gas is substantially nonreactive (inert) with the first precursor gas. In other embodiments, the radical species is generated from a second precursor gas that may be reactive with the first precursor gas.
While oxygen radicals are a highly reactive species for oxidation of certain metal precursors, such as trimethylaluminum (TMA) and titanium tetrachloride (TiCl4) for example, the present inventors have discovered that thin films deposited in an REALD process involving oxygen plasma generated from regular oxygen gas (O2) are inferior to thin films deposited by many other ALD methods. The inventors' experiments with ozone (O3) precursors have resulted in even poorer films, which suggests that direct oxygen plasmas formed from O2 are an inferior precursor for REALD because they contain a relatively high concentration of ozone—a gas that is much more persistent than oxygen radicals (free radicals) and therefore more likely to migrate into the second precursor zone and react with the metal precursor or other precursor introduced there, causing non-ALD deposition to occur.
The inventors have recognized these phenomena as an opportunity for improved REALD methods and improved methods of generating oxygen radicals for thin film deposition.